The present invention relates to a roots type fluid machine for transferring fluid by rotating a rotor.
A roots type pump (or roots type fluid machine) is widely used for a blower and a vacuum pump. A single stage roots pump shown in FIGS. 15, 16 has a pair of rotors 101A, 101B fixedly mounted on rotary shafts 102, 103 in a case 100, respectively. The rotor 101A is rotated by a drive gear (not shown) fixed on the rotary shaft 102 and the other rotor 101B is rotated in synchronization with the rotor 101A by the rotation of a driven gear (not shown) engaged with the drive gear. The pair of rotors 101A, 101B rotates synchronously in opposite directions with their lobes engaged with each other. Gas introduced through an inlet 105 by the synchronous rotation of the paired rotors 101A, 101B is trapped in a transfer chamber 110 formed by the case 100 and the rotors 101A, 101B. The gas is transferred from the inlet 105 to an outlet 106 of the roots pump in accordance with the rotation of the rotors 101A, 101B. Subsequently, the gas is released, e.g., by a later stage subsidiary pump.
Japanese Patent Publication NO. 2884067 discloses a roots type blower having a zigzag shaped groove formed in the inner wall of the blower case at a position adjacent to the blower outlet. When air flows back from the outlet, the zigzag groove decreases the air-flow velocity gradually while the air is flowing through the zigzag groove thereby to decrease the noise generated during the operation of the blower.
The roots type pump disclosed by the Japanese Patent Publication NO. 2884067 and shown in FIGS. 15 and 16 has clearances with predetermined dimensions (0.1-0.3 mm) between the rotors 101A and 101B and also between the case 100 and the respective rotors 101A, 101B. The roots type pump is configured so that the rotors 101A, 101B rotate while keeping the respective clearances. Since there is a pressure difference between the inlet 105 and the outlet 106 of the roots type pump, gas leaks through the clearances. Specifically, in the transfer chambers 110 formed by the case 100 and the respective rotors 101A, 101B of the roots type pump, gas leaks through the clearance formed along an inner wall 100A of the case 100 between the inner wall 100A and the respective rotor outer surfaces 101AA, 101BA, as indicated by arrow B in FIG. 15, and also through the clearance A formed in axial direction of the rotary shafts 102, 103 between a side wall 100B of the case 100 and the respective rotor end surfaces 101AB, 101BB, as indicated by arrow C in FIG. 16. The leakage through the clearance A connecting directly the outlet 106 on high-pressure side of the roots type pump and the inlet 105 on low pressure side thereof is a main factor for reducing the pump efficiency and hence causing an increase of power consumption.
The present invention is directed to providing a roots type fluid machine which can reduce the gas leakage through a clearance in axial direction of its rotary shaft between the discharge space and the suction space.